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Vintage Radio AWA 500M superhet mantel radio By Ian Batty The 500-series mantels were a ‘cheap and cheerful’ budget offering, released in four versions. They are tidy-looking sets that fit just about anywhere. I picked this one up at a Historical Radio Society of Australia (HRSA) auction some years back. A ppearing in 1946, the 500M was a well-tested design using all octalbased valves. It’s a compact set with little wasted space inside its Bakelite cabinet. The 500M is almost a conventional superheterodyne radio (‘superhet’). The difference – which I didn’t appreciate at first – is that it has only one audio stage. In other words, it has only three signal stages (see Fig.1). There are well-performing fourvalvers about, but they use audio reflexing in the intermediate frequency (IF) amplifier stage, giving it a dual role. In that case, there are effectively four signal stages (converter, IF amplification, audio preamplification, and audio output), like a typical domestic superhet. So this one is a bit unusual. 94 Silicon Chip The power supply uses a 6X5GT full-wave rectifier valve. The HT filter includes the electrodynamic speaker’s field coil and two 8μF electrolytic capacitors (C21/C22), forming a pi filter. The mains transformer provides two mains voltage tappings: 200~230V and 230~260V. C18 (100nF) provides RF/IF filtering for the common HT line; there is no decoupled/HT2 supply for the RF/ IF section. The converter uses a 6A8G, the octal pentagrid based on the original 2A7 and its follow-on 6A7. These earlier types were mounted on 7-pin UX bases. The converter has no self-bias, as its cathode returns directly to ground. Bias is supplied via the antenna Australia's electronics magazine circuit’s L3 from the back bias/AGC circuit. The screen grid supply is shared with the IF amplifier via dropper R3 and bypass C11. The antenna circuit uses an IF filter (L1/C1) which, unlike the Astor Mickey I reviewed in the January 2022 issue (siliconchip.au/Article/15179), causes little or no loss of sensitivity. The antenna circuit’s gain is improved at the top end by top-coupling capacitor C2, also known as a ‘gimmick’ capacitor. The antenna coil’s L2 primary ‘steps up’ to the tuned L3 secondary, giving a voltage gain of around three times. As L3 has no adjustable slug, this set’s RF alignment is done by adjusting the LO coil’s tuned winding (L4) to meet siliconchip.com.au siliconchip.com.au Fig.1: the circuit diagram for the AWA 500M. The radio has a standard IF of 455kHz. Interestingly the original service manual has separate listings for the 500M-Z and 500M-Z, with the 500M & 500-M-Z using a 40Hz transformer (T2), while the 500M-Z used a 50Hz transformer with a directly-heated 5Y3GT rectifier valve. L3 at 600kHz – more on that later. The local oscillator uses the ‘Armstrong’ design, with untuned primary L5 feeding back to its L4 tuned secondary. The tuning gang uses identical sections, so padder C8 ensures local oscillator/antenna circuit tracking. Grid resistor R1 returns to the cathode as usual – that just happens to be ground in this set. The converter feeds its IF signal to the slug-tuned first IF transformer primary L6. The transformer comprises L6/L7, with both windings tuned. The secondary, L7, feeds the 6G8G IF amplifier. This duo-diode pentode is commonly used either for IF or first audio stage amplification, with its diodes operating separately as the demodulator and for AGC, or combined (as here) demodulator/AGC. As with the converter, the IF amplifier has no self-bias; it’s biased (via L7) from the back bias/AGC circuit. The IF amp feeds its signal to the second IF transformer primary, L8. Its secondary L9 feeds the demodulator/AGC diodes in the 6G8G. Both transformer windings are slug-tuned. Demodulated audio, and a DC voltage proportional to the incoming signal, are developed across volume control R7. Audio is taken from R7’s wiper and fed via C17 to the output amplifier grid. The DC voltage across R7 is fed, via R4, to the AGC line. This has a standing bias of about -2V, derived from 40W back-bias resistor R6 via R5. This supplies bias to the converter and IF amplifier, which lack individual biasing circuits. The AGC voltage develops across volume control R7 and audio is filtered out by C4. It’s applied to the control grids of the converter and IF amplifier via the R4/R5 divider. This simple circuit has no effective delay, with a measurable AGC voltage for an input signal of only 100μV. The 6V6GT output stage uses cathode bias. Be aware that the near-­ identical 500s used back bias for all its valves. Audio, fed from the volume control, is applied to the control grid via R9. This ‘stopper’ resistor reduces the high-gain 6V6GT’s tendency to oscillate. Its anode feeds the primary of output transformer T1, bypassed by C20. This capacitor suppresses the output transformer’s natural resonance caused by its combined winding inductance and capacitance. Australia's electronics magazine September 2023  95 forms. These initial releases were given the “G” (glass) suffix (6A8G, 6G8G etc). They used a flattened ‘press’ at the bottom of the envelope to seal the lead-in wires, as with the previous 4-to-7-pin UX construction. Fig.2 shows 2V/1.5V pentagrid converter development from the initial 1C6 issue to the final 1A7 that preceded the all-glass 1R5. With the push towards compact equipment, manufacturers simplified the glass envelope and released tubular types. The original metal types had short lead wires between the base pins and the internal elements. The G and original GT types used press construction, so they were quite tall compared to metal equivalents. Also, they did not perform as well The dial markings are painted onto a fancy-looking piece of cloth. Another at higher frequencies due to extra lead separate piece of this cloth is then used as a speaker grille. inductance and capacitance. Notably, the high-performance 6AB7/6AC7 Notice that C20 is connected from guided by their invention of all-metal ‘video pentodes’ were not generally the anode to ground, giving it a stand- valves. These committed pin 1 to released in glass envelopes. ing voltage of some 230V. Should it grounding the metal shell/envelope, The Bantal (‘bantam-octal’) line go short-circuit, it will ground T1’s both for signal shielding and electri- reduced the envelope’s overall height anode connection, possibly burning cal safety in case of internal anode- by lowering the height of the press. out the output transformer. It’s best to shell leakage. Some Bantals used a metal shell to reconnect the capacitor so it’s across This meant that initially, only seven secure the envelope to a disc-shaped the output transformer’s primary. If it pin connections were available, so base; others simply continued with does short out, the only effect will be some valves (twin triodes such as the the Bakelite ‘bucket’. Fig.2 shows one a lack of audio. 6SN7) could not be released in metal of each: a 1A7GT and an equivalent V3’s 315W cathode bias resistor envelopes. 1A7GT(M). is a parallel pair of 630W resistors. While you can use a metal valve to Confusingly, some were initially These are the original fitment but of a replace a glass type, be sure that the set denoted GT/G or G/GT. Many types non-standard value; the nearest E12 manufacturer has not used pin 1 as an were never issued in the intermedi(10%) values are 560W and 680W. HT tie point; the metal valve envelope ate ‘long envelope’ style (the 1A7G The E24 (5%) series does have a 620W will be at (dangerous) HT potential! It example) but went directly from value, so maybe AWA just went off on has happened! the stepped tubular (‘G’) form to the their own with the 630W. Glass-envelope octals were orig- reduced-height GT cylindrical form inally released in the ST (stepped-­ (the 1A7GT). The 6V6GT and 6X5GT G, GT and GT/G valves tubular ‘coke bottle’) form previously in this set both used the reducedRCA’s design of the octal valve was used by the 4-, 5-, 6- and 7-pin UX height construction. Left: the rear of the AWA 500M chassis. Below: the grommet-and-knotted cord fitting on the underside of the chassis is not a very safe arrangement. 96 Silicon Chip Australia's electronics magazine siliconchip.com.au Eventually, ‘GT’ was applied to all tubular-envelope octals, regardless of base construction. Restoration The Bakelite case was in good condition, only needing a polish to restore it. Electrically, it was also in good condition, having been previously restored. All three electrolytic capacitors (HT filters, output cathode bypass) had been replaced, as had the paper types. All low-capacitance mica types were still in place. These are generally more reliable, but are known to suffer leakage over time due either to internal dendritic (‘metal whisker’) growth, or (as mica is hydrophilic) from gradual moisture absorption. A fellow HRSA member once reported a radio with a mysterious ‘crackling’ sound. The fault was traced to intermittent leakage in the mica capacitor bypassing the first audio amplifier’s anode to ground. How good is it? At first, I thought it was pretty poor. But looking at the circuit reminded me that I had not fully appreciated its budget design. Thinking about the Astor Mickey, I’d fallen into the trap of expecting tens of microvolts sensitivity at worst. Adding a first audio stage, with a gain of maybe 50 times, would easily have given the performance I’d had in mind. I went stage-by-stage and measured the signal needed at each grid to get the standard 50mW output. I use two Fig.2: examples of different types of glass-envelope and tubular-envelope valves. The base and envelope both evolved to produce more compact valves. references: my own testing and my preferred servicing manual for this class of radio, Markus and Levy’s Elements of Radio Servicing. If you don’t have a copy, I suggest you get one. The output stage needed around 500mV at its grid to give a 50mW output. I test at 400Hz, as I’ve found some sets that begin cutting off at 1kHz! Going to the IF amp’s grid, I needed 25mV of 400Hz modulated signal for 50mW of output power. The converter grid needed 1.5mV at 600kHz and 1400kHz. For the standard 50mW output, it needed 500μV at 600kHz or 400μV at 1400kHz injected into the antenna. Due to its low gain, the signal-plus-noise-to-noise ratio (S+N:NR) exceeded 20dB in both cases. These figures are consistent with Markus & Levy’s and my own experience. The audio output was about 1.5W at clipping. At 50mW, Total harmonic distortion (THD) was 3%. Audio response from the volume control to the speaker was 170~1500Hz, but from the antenna to the speaker, it was only 190~900Hz. The IF bandwidth at -3dB was ±2.9kHz and ±30kHz at -60dB. AGC action was only moderate, with a 20dB input signal increase giving a 6dB rise in output level. That results from the R4/R5 circuit combining the back bias and AGC voltages. For a The underside of the chassis. Very little was required to polish up the radio, as the electrolytic and papertype capacitors had already been replaced. Note the use of a cord anchor to replace the original and unsafe knotted cord. Australia's electronics magazine September 2023  97 strong signal of 100mV at the input, around -40V is developed across volume pot R7 but only about -11V is conveyed to the AGC line. Also, the ‘undelayed’ AGC cuts in early. At 1400kHz, I needed 400μV at the antenna terminal for 50mW output, but shorting the AGC to ground cut the required input signal level to only 270μV, a sensitivity increase of some 3.5dB. This is moderate sensitivity by any measure, but my 500M is a budget set with three signal stages. You’d expect to use it with a few metres of antenna wire connected. With that, all Melbourne stations rocked in, and I was able to get my distant station, 3WV, at a reasonable volume with just a 2m-long antenna. All in all, it’s a simple mantel set without any pretensions. It’s also a straightforward design that’s easy to work on and fix. Hint on LO testing If a superhet’s local oscillator is not working, the set will do nothing, but many other faults can result in no audio output. So, if the set is not functional, how can you be sure the LO is OK? Some repairers measure the oscillator’s negative grid voltage. I was able to do this with the 500M (as noted on the circuit diagram), but with most sets I’ve tried this on, the LO stops due to the extra loading on the circuit. My preferred method is to use a good set as a monitor, tuned to the top end of the band (this works for any superhet – valve or transistor – on any band). Slowly tune the suspect set from the bottom up towards the top of the band. For the broadcast band, you’d tune the monitor set to the top end at 1600kHz. Assuming an IF of around 450kHz, the suspect set should produce a ‘swoosh’ or ‘birdies’ in the monitor at around 1150kHz on the suspect set’s dial. If the suspect set is a really old one with a 175kHz IF, expect a response from the monitor just above 1400kHz on the suspect set’s dial. As a bonus, you don’t even have to take the suspect set out of its cabinet/ case! Is it worth buying one? If you see a 500M, don’t be put off by its modest performance – it’s a nice-looking set with a compact design that lets it sit anywhere and provide entertainment. Radiolette ‘500’ versions I could not find a model identifier on my set – you may need to pull the chassis and inspect the wiring to discover whether you have the ‘all back bias’ version or its alternative with cathode bias on the output. There are several 500Ms. Kevin Chant’s listing for the 500MY uses back bias for all valves. AGE also released the set as their G64ME. Radiomuseum lists two circuits: 500M and 500M-Z, both identical and applicable to the 500M, 500M-Z and 506. These show cathode bias for the output stage and an alternative power supply using a directly-heated 5Y3GT/G in the 500M-Z. Special handling It’s an easy set to work on but heavier than I expected, probably due to the combination of the electrodynamic speaker and a larger-than-expected power transformer. The VE301 (February 2023 issue; siliconchip.au/Article/15671), had no mains cord security – the active lead had actually broken off and was floating about under the chassis and had to be fixed! My 500M had the commonly-­ used (and unsafe) grommet-­ a ndknotted cord fitting. References & links • Marcus, A. H., & Levy, W. H, “Elements of Radio Servicing”, McGrawHill Book Company, Inc. (1947). • Radiomuseum AWA 500M-Z: siliconchip.au/link/ablq • Kevin Chant’s website, under 500MY: siliconchip.au/link/ablp • Verrall, Bill, “The AWA Radiolette Model 500MY”, Radio Waves, HRSA, Issue 84, April 2003, p8. Bill’s article SC has a parts layout diagram. From the side you can see the 6C8G valve has the label “goat patented” on it. These Goat Shields were very common in the 1940s-50s and went out of use when the straight-sided glass tubular (GT) forms came into use. 98 Silicon Chip Australia's electronics magazine siliconchip.com.au